Hi crazymommaj I have been growing camellia here for a few months my self i started from seed though. wasn't very difficult at all to do and i ended up with very strong plants. However, i have been experiencing some yellowing my self but more of a leaf spotting green with yellow spots. i believe it might be close to what you are having happen. i tested my soil PH and it looked like it was at a 6 and 1/2 and it needs to be at 4-5 PH they like to be more on the acidic side and if they are not they will start yellowing. so i am going to mix up with this next watering some cold coffee make sure its COLD not hot. just pour a cup in there once ever two weeks or once every time you add a fair amount of water to the pot. it will balance the PH and make it just right. if you would like i have a youtube channel you could subscribe i will be doing a video on the yellowing this week and if my coffee experiment works with mine.

Here is a link that might be useful: TheItalian Garden

There are a lot of variables that could be at play here, but you ask if the potting soil could be the problem, and I think it could be at least part of the problem. Osmocote is pretty high in nitrogen which is designed to promote foliage and flowers for established plants, but not root growth. For very young plants a low-nitrogen (or no nitrogen) fertilizer is recommended.

Or better still, an slow-release organic source of nutrients such as compost/cotton meal/liquid seaweed or an organic fertilizer (Holly-tone is an easy-to-find choice for camellias and other acid-loving plants).

Also, I think potting mixes probably won't provide a sufficiently acidic environment for camellias. I may be mistaken, but I think most potting mixes are fairly close to a neutral pH. You can easily test the pH of the mix with some litmus paper (available in soil testing kits if you don't happen to have litmus paper lying around the house).

Good luck!